This application claims priority from Korean patent application No. 2002-09879 filed Feb. 25, 2002 in the name of Samsung Electronics Co., Ltd., which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method for making semiconductor integrated circuit devices, and more particularly, to a method for sawing a wafer.
2. Description of the Related Art
An image sensor device, which is one example of a semiconductor integrated circuit device, has a large number of pixels that convert light into electric signals. A charge-coupled device (CCD) is one such image sensor device. In the CCD, images are obtained when incident light, in the form of photons, falls on the array of pixels. An electronic charge converted from the photon is transferred within a silicon substrate and causes a voltage to be formed. Recently, a CMOS image sensor has been introduced, which is slightly inferior to the CCD in image quality but has a smaller size and lower power consumption.
In such image sensors, most chip surfaces are active regions for detecting images. Thus, the quality of the image sensors and sensitivity depends on its fabrication process. For this reason, the greater pixel density required of the image sensor devices, the greater reduction in the yield of the image sensor devices, presenting significant obstacles to the development of the image sensor devices.
The problems that may arise in the image sensor devices are image defects such as a black defect, a white defect and/or a dark defect. While the black defect is displayed as a black dot on a screen when a signal output of a pixel is below the standard (or the average) illuminance of 4 lux, the white defect is displayed as a white dot under opposite conditions. In addition, a dark defect is displayed as a white dot by thermal generation at zero lux.
The above-mentioned image defects, especially a black defect, may be caused by silicon dust produced in a wafer sawing process. The wafer sawing process and the subsequent die-attach process for a wafer and the effect of silicon dust are explained as follows.
Referring to FIGS. 1 and 2, a wafer 10 that is composed of a large number of CCD-type image sensor devices 11 is diced or singulated along scribe lines 31 by a rotating wheel blade 51, and thereby separated into individual image sensor devices 11. This is generally known as a wafer sawing or dicing process. The backside surface of the wafer 10 adheres to an adhesive tape 27 before wafer sawing, and the separated individual devices 11 stay on the adhesive tape 27 even after wafer sawing. The wafer sawing process, however, produces silicon dust 41 that may cause the aforementioned image defects. Therefore, to prevent the silicon dust from remaining on the surface of the wafer 10, a spray nozzle 53 sprays a cleaning solution 57 on the wafer 10 at high pressure. The silicon dust 41 is thus detached from the surface of the wafer 10 and drawn into a suction pipe 55.
The wafer 10 is then moved to a location for a die-attach process. In the die-attach process, a pickup collet 59 picks up one of the individual devices 11 by vacuum force and attaches it to a substrate.
As described above, the wafer sawing process includes removing silicon dust 41 from the surface of the wafer 10 by using the spray nozzle 53 and the suction pipe 55. However, as shown in FIG. 2, such removing action cannot eliminate the silicon dust 41a staying in the gap 33 between the adjacent individual devices 11, particularly the dust adhering to the adhesive tape 27 in the gap 33. The non-removed silicon dust 41a may contaminate the micro-lenses of the device 11 and block the light incident upon the lenses, thereby causing the image defects. This can substantially degrades the display quality. This undesirable phenomenon often occurs when the individual device 11 is detached from the adhesive tape 27 by the pickup collet 59 for the die-attach process.
For a wafer having a thickness of 680 micrometers, a 5xcx9c6 percent reduction in yield can be caused by silicon dust during the wafer sawing process, whereas a 8 percent reduction in yield can be caused by the silicon dust during the pickup action for the die-attach process. Therefore, the image sensor devices require more attention to prevent them from being contaminated by the silicon dust than the other semiconductor devices such as memory devices.
In one embodiment, a wafer having semiconductor devices, e.g., image sensor devices is provided. Each image sensor device has a large number of micro-lenses formed thereon. A protective layer is formed on an active surface of the wafer such that the protective layer covers the micro-lenses. A first adhesive tape is attached to a backside surface of the wafer. Then, the wafer is diced into individual image sensor devices. A second adhesive tape is attached to the protective layer on the active surface of the wafer. The first adhesive tape is removed from the backside surface of the wafer. A third adhesive tape is attached to the backside surface of the wafer. Subsequently, the second adhesive tape is stripped from the active surface of the wafer.